1. Field Of the Invention
This invention relates generally to the design of the structure of the magnetic core for high frequency alternators used to generate electrical power, and more particularly to a low loss core of the claw pole type alternator.
2. Description of the Prior Art
The production of electrical power from high frequency alternators allows small units to generate high levels of power output due to the rapid change of flux. The small size and weight of high frequency units conveys numerous benefits in the design, manufacture and cost of power systems. Furthermore, the output of high frequency alternators, when coupled with solid state controls, can produce high quality DC and low frequency AC power relatively independent of the speed of the driving engine. This confers numerous other system benefits, including optimization of engine speed for minimum fuel consumption, or generation of power incidental to operation of a prime mover.
The claw pole alternator structure provides a large number of field poles of alternating polarity excited by a single rotor coil. The large number of poles and the alternating polarity of adjacent poles in the claw pole design combine to produce the high frequency change of flux needed for high frequency power production and small size.
Generally, laminated magnetic materials are used in the magnetic pathways where flux varies rapidly. This dramatically reduces the losses due to eddy currents caused by the varying flux. Magnetic and electrical steels are used to minimize the hysteresis losses caused by flux variation. Typically the laminated materials are used in the stator structure that surrounds the claw pole rotor because high frequency alternators are designed for the flux to fluctuate in the armature poles located on the stator. Because the claw pole rotor of a typical high frequency alternator is normally subjected to quasi-DC excitation, it is possible and convenient to manufacture the rotor from solid iron. Solid iron rotors are relatively simple to manufacture, and allow for automatic shifting of the flux distribution within the rotor during rotation.
However, for applications involving the modulated excitation of the field, such as detailed by Hilgendorf in U.S. Pat. No. 3,916,284, there are large core losses (eddy current losses and hysteresis losses) in the solid iron rotor. Furthermore, even for quasi-DC excitation, at high shaft RPM the shifting of flux under the armature poles causes very high core losses in the tips of the (solid) rotor claws. ("Slotting harmonics of the air-gap field induce eddy currents into the claws . . . . It was found that the ratio of iron losses (rotor to stator) is about 2:1"--Kuppers and Henneberger, "Numerical Procedures for the Calculation and Design of Automotive Alternators", IEEE Transactions on Magnetics, vol. 33, no. Mar. 2, 1997.) These losses grow with increasing frequency, and become a greater percentage of the overall losses as the frequency increases. Tests by Kuppers and Henneberger show that for a typical claw pole alternator operating at 10,000 shaft RPM the core losses in the solid iron core become a full third of the total losses. These losses are also the fastest growing portion of losses as the shaft speed increases further. These rotor core losses become a limiting factor in increasing the alternator frequency in order to reduce size and weight.